1. Field of the Invention
The present invention relates generally to pressure to electrical signal transducers or transmitters, and relates specifically to that type of such transmitters which employs a differential pressure sensor or sensing element to produce an electrical signal of a value dependent upon the difference between two fluid pressures. Transmitters of this type are referred to herein as differential pressure transmitters. More specifically, the invention relates to transmitters of this type which employ barrier diaphragms and fill fluids for transmitting fluid pressures to the sensor.
2. Description of the Prior Art
Differential pressure transmitters of the form last noted above are known in the art. An example of a first type of such a transmitter, employing a sensor which has only a minute displacement over its operating range, is shown in the Weaver et al U.S. Pat. No. 3,712,143. In the latter transmitter, the sensor is a semiconductor strain-sensitive wafer. An example of a second type of such a transmitter, employing a sensor having a relatively large displacement over its operating range, is shown in the Frick U.S. Pat. No. 3,618,390.
Differential pressure transmitters of the form shown in the above-noted patents include a so-called head structure which contains two barrier diaphragms. A chamber on one side of one diaphragm is pressurized with a fluid having a first pressure, and a chamber on one side of the other diaphragm is pressurized with a fluid having a second pressure. These two fluids, hereinafter referred to as applied fluids, may well be so-called process fluids which are on the two sides of an orifice plate in a process flow line.
A separate chamber on the other side of each of the diaphragms contains a fill fluid. These fill fluids communicate with the opposite sides of the sensor, and the latter is displaced or deflected by an amount which is dependent upon the difference between the two pressures of the fill fluids, and which is thus dependent upon the differential pressure of the two applied fluids. The sensor produces an electrical output signal which is dependent upon said displacement and hence upon said differential pressure. In all of such known transmitters with which I am familiar, the sensor is located within the head structure adjacent the barrier diaphragms.
Although the known transmitters, as just described, have provided the desired results in many applications, they have exhibited shortcomings in two areas. Thus, because of the minute full range displacement of the sensor in the transmitters of said first type, it has been necessary to equip such transmitters with overload protection arrangements to prevent the sensors from being excessively displaced, and hence degraded, damaged, or destroyed, by excessive applied fluid pressures. In the transmitter of the above-noted Weaver et al patent, such overload protection is provided by a bellows-actuated valve arrangement. Such an arrangement increases the size, complexity, and cost of the transmitter to an extent which is undesirable in certain cases.
In the transmitters of said second type, overload protection is more readily obtained, but the relatively large displacement sensors of those transmitters have been found to be less desirable than the minute displacement type of sensor employed in said first type of transmitter.
The other area in which the known transmitters have exhibited a shortcoming is that wherein there are applied fluids present on the outsides of the two barrier diaphragms which are at high temperatures and/or are subject to pulsations. It has been found that the barrier diaphragms and the fill fluids tend to transmit the applied fluid heat and/or pulsations to the sensor because of the latter's location within the head structure which is supporting the barrier diaphragms. The result is that the sensor is either rendered inaccurate or damaged.
Additionally, the location of the sensor in the head structure requires a fluid tight seal at the point where the electrical output leads from the sensor pass out through the wall of the head structure. Since such a seal must be capable of withstanding the heat of the process fluids, this seal requires the use of a relatively expensive material.